1. Field of the Invention
The present invention relates to a nitride semiconductor device, used for power electrics application and the like, for instance.
2. Description of the Related Art
Conventionally, a nitride semiconductor device using a nitride has been known. Among such nitride semiconductor devices, a nitride semiconductor device using GaN has a wider band gap compared with a semiconductor device using Si. Accordingly, a device having a high critical electric field and being small in size and high in breakdown voltage is easy to be put into practice. With such a device, in semiconductor devices for power electronics application, a device with low on-state resistance and low-loss can be realized. Especially, a heterostructure field-effect transistor (HFET) using an AlGaN/GaN heterostructure can obtain an excellent characteristic with a simple device structure.
As a nitride semiconductor device having such an AlGaN/GaN heterostructure, one formed of the AlGaN/GaN heterostructure by crystal-growing on a substrate of sapphire, silicon carbide (SiC), or the like has been known (refer to Japanese Patent Laid-Open Application No. 2001-168111, for instance).
As mentioned above, in a conventional nitride semiconductor device, the AlGaN/GaN heterostructure is generally formed by crystal-growing on a substrate of sapphire, silicon carbide (SiC), or the like. Since these substrate materials have lattice constants comparatively close to that of AlGaN/GaN hetero structure, it is possible to crystal-grow film of several μm in thickness without cracks.
However, since a sapphire substrate has large thermal resistance, it has a disadvantage in that thermal radiation from the devices is difficult. On the other hand, since a SiC substrate is small in thermal resistance, it has no problem in the thermal radiation but has a disadvantage in that it is difficult to obtain a substrate with a large diameter, and even the substrate having a small diameter of 2 or 3 inches is high-priced.
Accordingly, it is conceivable to form a AlGaN/GaN heterostructure using an inexpensive and less thermal resistant silicon (Si) substrate which is easily obtainable in a large diameter to manufacture a nitride semiconductor device. However, Si and the AlGaN/GaN heterostructure differ largely in lattice constant. Therefore, cracks due to deformation is easy to generate, and on the Si substrate, only a GaN layer about 1 to about 2 μm, in thickness can be obtained by crystal growth without generating cracks. And it has a disadvantage in that since the upper limit in breakdown voltage of the GaN device on the Si substrate is determined by the thickness of the GaN layer, sufficient breakdown voltage cannot be obtained.
In a semiconductor device used for power electronics application, avalanche withstanding capability is required to be able to let electric current flow without the device being destroyed even when high voltage equal to or greater than static breakdown voltage is applied to the device to cause avalanche breakdown. In order to realize high avalanche withstanding capability, it is necessary to rapidly perform discharge of holes which are generated at the time of avalanche breakdown. For this purpose, a high concentration p-type doped layer is indispensable. However, in the nitride semiconductor device, since the activation rate of the p-type dopant is low, there is a disadvantage in that both the formation of a high-concentration p-type layer and the realizing of a high avalanche withstanding capability are difficult.